1. Field of the Invention
The present invention relates to an image forming apparatus and control method thereof and, more particularly, to an image forming apparatus which prints an image by an inkjet method, and a control method thereof.
2. Description of the Related Art
With regard to most inks conventionally used to print an image by an inkjet method, a dye which easily dissolves in water serving as a main component is adapted as a color material. The color materials of most the dye inks easily penetrate into a print medium together with water. Even when an image is printed on a glossy medium with a smooth surface, the color material penetrates into the medium, maintaining the smoothness of the medium surface and obtaining a highly glossy printed material. Technical problems when forming a glossy image have been solved by improving print media.
Recently, demands arise for further improvement of light resistance and water resistance of printed materials. To meet these demands, ink which adopts a pigment as a color material is being developed. In general, however, the color materials of most pigment inks do not satisfactorily penetrate into a print medium. The fixing characteristic or glossiness of a print medium has room for improvement.
The fixing characteristic can be improved to a certain extent by employing, e.g., multipass printing. A technique of improving the fixing characteristic by multipass printing will be described.
FIG. 1 is a schematic view for explaining multipass printing. Multipass printing is generally adopted in a serial inkjet printing apparatus. Multipass printing is to form an image stepwise by multiple print scans in the same image region of a print medium.
In FIG. 1, reference numeral 201 denotes a state of a print medium after the first print scan. “1A” represents a dot landed by the first print scan. In this example, dots 1A are printed without overlapping each other. Reference numeral 202 denotes a state of the print medium after the second print scan. “2A” represents a dot landed by the second print scan. Similarly, reference numeral 203 denotes a state of the print medium after the third print scan; and 204, a state of the print medium after the fourth print scan. “3A” and “4A” represent dots landed by the third and fourth print scans, respectively. Printing in the same image region is complete by the four print scans represented by the states 201 to 204. Positions printable by the respective print scans are determined by AND processing using print data and binary data called a mask pattern, and the like.
In multipass printing, since a print medium is conveyed between print scans, and ink droplets are applied to the print medium with a predetermined time difference. Printing can progress while applied ink droplets dry even on a print medium whose pigment ink absorption speed is low, such as plain paper. This results in a good fixing characteristic.
Since a print medium is conveyed between print scans, different printing elements print in the same image region in the respective print scans. Even if discharge from each printing element varies, the variations can be distributed and made less conspicuous. At the boundary between print scans, so-called white streak or black streak may appear owing to variations in the amount of conveyance. Multipass printing can make such a streak unnoticeable. Discharge variations of each printing element and variations in the amount of conveyance are image degradation factors which are inevitable owing to the manufacturing process and precision. Hence, the above-described multipass printing is an important printing technique for maintaining the image quality by a serial inkjet printing apparatus, and is popularly employed.
However, the glossiness of a printed portion may be impaired when performing multipass printing on a print medium whose surface has undergone special processing, such as glossy paper.
Generally, a print medium such as glossy paper has small pores in the surface to absorb an ink solvent and improve the fixing characteristic of a color material. A print medium can absorb the dye of dye ink via pores together with water. However, the pigment molecules of pigment ink hardly dissolve in water, and disperse as fine particles in water. Since the fine particle is larger than the pore on the medium surface, the color material hardly penetrates into the print medium. In other words, the fine particles of pigment are deposited and fixed on the surface of the print medium. As a result, the smoothness of the surface of the print medium is impaired, losing glossiness.
When multipass printing uses pigment ink whose color material hardly penetrates into a print medium, dots applied by print scans sequentially dry on a print medium, overlap each other, and are fixed. In four times multipass printing shown in FIG. 1, four ink layers overlap each other. Contrary, when an entire image is complete by one print scan without adopting multipass printing, only one ink layer exists. For this reason, the surface of a print medium becomes rough and readily loses glossiness in a case where multipass printing is employed, compared to a case where no multipass printing is employed.
To solve these problems, several solutions are proposed. For example, there is disclosed a technique for improving glossiness by applying ink to a print medium and fixing it by a heat roller (see, e.g., Japanese Patent Laid-Open No. 2001-200183).
There is also disclosed a technique for preparing pigment ink containing a photosetting monomer or oligomer, applying the ink to a print medium, and irradiating the print medium with ultraviolet light or the like to set the ink (see, e.g., Japanese Patent Laid-Open No. 2001-323192). According to this technique, a resin film is formed on the ink surface upon irradiation with ultraviolet light or the like, and improves surface smoothness and glossiness.
However, to implement the above-mentioned fixing by a heat roller and ultraviolet irradiation, the printing apparatus requires a component such as a heat roller for heating a print material or a light-emitting means for emitting ultraviolet light or the like. This raises the cost of the printing apparatus and complicates the image forming process.
To solve this, there is disclosed a technique for obtaining glossiness suitable for each print medium by selecting the number of passes or a mask pattern in accordance with the glossiness of a print medium in an inkjet printing apparatus using pigment ink (see, e.g., Japanese Patent Laid-Open No. 2005-297212). According to this technique, satisfactory glossiness can be maintained without performing post-processing such as fixing by a heat roller or ultraviolet irradiation, and without impairing the glossiness of a printed portion on a glossy print medium as much as possible (see, e.g., Japanese Patent Laid-Open No. 2005-297212).
However, the technique described in Japanese Patent Laid-Open No. 2005-297212 aims at improving the glossiness of a formed image. It is, therefore, difficult to obtain uniform glossy of an entire image owing to a gloss characteristic unique to each ink and the shape difference in fixing. This technique cannot suppress so-called gloss unevenness.